1. Field of Invention
The present invention relates to technology for a switching power supply and peripheral circuitry for a switching power supply, and relates more particularly to technology for a switching power supply device, a semiconductor device, and a control method for a switching power supply device.
2. Description of Related Art
Switching power supply devices conventionally require an overload protection function in order to prevent high current output in the event a shorted load or other load problem causes an output voltage drop, and to prevent current exceeding a predetermined level from accidentally flowing to an electronic device load. Numerous designs affording such an overload protection function are known from the literature.
Providing an overload protection function requires solving two problems, (1) how to detect an overload state, and (2) how to provide protection, and various designs of solving these problems are known from the literature.
There are three ways of detecting an overload as described below with reference to FIG. 12.
The switching power supply shown in FIG. 12 includes a switching device 201, a control circuit 202 that has CC, FB, and GND pins and controls the switching device 201, and a transformer 231, and detects the output voltage VO by resistances 246 and 247, shunt regulator 245, and photocoupler 232. This switching power supply changes the current flow from the FB pin of the control circuit 202 based on the output voltage VO, and switching device 201 and the first coil 231a of the transformer 231 adjust the energy supplied to the output. The switching power supply shown in FIG. 12 is a common rated voltage supply device. The secondary coil 231b and auxiliary coil 231c of the transformer 231 have the same polarity, and the auxiliary supply voltage VCC of the CC pin that is smoothed by diode 204 and capacitor 205 is ideally proportional to the output voltage VO. The CC pin is also commonly used to supply power to the control circuit 202 during normal operation.
A first arrangement for (1) detecting an overload state monitors the energy supplied by the switching device 201 and transformer 231 to the output and detects an overload when the supplied energy exceeds a predetermined level. More specifically, an overload is detected when the peak current flow through the switching device 201 reaches a predetermined level. PWM (pulse width modulation) control and ringing choke converter control methods determine the output power by controlling the time current flows to the switching device 201. This output power increases linearly to the peak level of current flowing through the switching device 201, and this can be used to detect an overload state when the output power and output current IO exceed a predetermined level.
A second arrangement uses the auxiliary supply voltage VCC that is smoothed and generated proportionally to the output voltage VO by the secondary coil 231b and auxiliary coil 231c of the same polarity. An overload is detected when the auxiliary supply voltage VCC drops below a predetermined level. The maximum energy level supplied to the output is determined by setting the peak current flow through the switching device 201 with PWM control, for example, and by setting the maximum oscillation frequency with PFM control. Because the auxiliary supply voltage VCC drops as the output voltage VO drops as the load increases further, the auxiliary supply voltage VCC output from the CC pin can be used to detect an overload when the output current IO exceeds a predetermined level.
A third arrangement likewise uses the secondary coil 231b and auxiliary coil 231c of the same polarity to detect an overload when the auxiliary supply voltage VCC exceeds a predetermined level. As noted above, the auxiliary supply voltage VCC is ideally proportional to the output voltage VO. As the load increases, the energy supplied from the secondary coil 231b to the load increases, and the output power increases. However, because the power consumed by the control circuit 202 at the CC pin is substantially constant, the energy supplied from the auxiliary coil 231c to the control circuit 202 is also constant. As a result, the auxiliary supply voltage VCC tends to rises as the load increases. Using this increase in the auxiliary supply voltage VCC, an overload is detected when the output power and output current IO exceed a predetermined level.
There are generally two types of arrangements for (2) providing protection, latch off and auto-recovery. Latch-off methods stop the switching operation (referred to below as “oscillation”) of the switching device when an overload is detected and prevents the switching device from oscillating until the input voltage drops. Self-resetting arrangements enable the power supply to operate normally again once the overload state is cleared after an overload is detected and the protection circuit operates.
One type of auto-recovery arrangement is the foldback current limiting circuit that reduces energy supply to the output as the output voltage VO drops when an overload is detected so that the output current IO drops as the output voltage drops indicated by the output voltage-output current (V-I) curve shown in FIG. 13A.
A second auto-recovery arrangement is the simple current limiting circuit having the output voltage-output current (V-I) curve shown in FIG. 13B, that is, the output current IO remains constant while the output voltage drops at a certain threshold current.
A third auto-recovery arrangement is the intermittent timer switching circuit that reduces the oscillation period of the switching device at a constant rate when an overload is detected to increase the time oscillation is off and reduce the energy supply to the output.
Japanese Unexamined Patent Appl. Pub. H5-68330, for example, teaches an auto-recovery overload protection device that detects an overload from a drop in the smoothed voltage of the auxiliary coil and limits current output.
Japanese Unexamined Patent Appl. Pub. H6-153382 teaches a latch off overload protection device that detects an overload when current flow to the switching device exceeds a predetermined level.
In switching power supplies the control circuit controlling the switching device is often provided on a semiconductor substrate, or the switching device and the switching device control circuit are provided on the same semiconductor substrate. These semiconductor devices are sold as a control IC for a switching power supply, and make it easier to design a switching power supply.
As noted above, both latch off and auto-recovery designs can be used for overload protection. However, while one power supply device may require latch-off overload protection in order to assure a safe shutdown when an overload occurs, other power supply devices require auto-recovery protection enabling the power supply to restart automatically when the overload state is resolved after the overload protection operates when an overload is detected. Both types of overload protection have their advantages and disadvantages depending upon the devices connected to the power supply output and the operating environment.
However, only one type of overload protection is rendered on this type of switching power supply control IC, and when a control IC (semiconductor device) is used when designing a switching power supply, the type of overload protection cannot be selected. The utility of the control IC is thus diminished.
The present invention solves this problem by rendering a plurality of arrangements for detecting an overload state, such as an arrangement for detecting when the energy supplied to the output through a transformer is greater than or equal to a predetermined level and an arrangement for detecting when the smoothed voltage from the secondary coil of the transformer and the auxiliary coil of the same polarity drops, and operating a switching device to select one of the arrangements. Latch off overload protection is applied when one of these detection arrangements detects an overload, and auto-recovery overload protection is applied when an overload is detected by the other arrangement.
The invention also provides a control IC (semiconductor device) having this functionality, and provides a switching power supply device that uses this control IC and has the peripheral circuits for implementing latch-off type overload protection and auto-recovery type overload protection.